The present invention relates to programmable controllers which are employed to control the operation of one or more pieces of manufacturing equipment, and more particularly to modules which interface the programmable controller to remote devices using a communication link.
Programmable controllers, such as the one described in U.S. Pat. No. 4,858,101, typically are connected to industrial equipment, such as assembly lines and machine tools, to sequentially operate the equipment in accordance with a stored control program. The control program includes instructions which are read out in rapid sequence and executed to examine the condition of selected sensing devices on the controlled equipment, or to energize or deenergize selected operating devices on the controlled equipment contingent upon the status of one or more of the examined sensing devices.
Many types of sensing devices have been devised for detecting various conditions on the controlled equipment and providing an input signal to the programmable controller. The simplest of these types of devices are switches which send a DC or an AC signal to the programmable controller. Other devices, such as temperature or pressure sensors provide a four to twenty milliampere signal having a magnitude which corresponds to the magnitude of the condition being sensed. Similarly with respect to the operating devices, the programmable controller must be capable of providing a variety of electrical output signals either DC, AC or analog current signals for driving the different types of operating devices on the controlled equipment. In order to provide a programmable controller which can be interfaced to the different types of input and output electrical signals, modularized controllers have been developed which allow the user to place different combinations of input and output modules into the programmable controller to interface with signals for the specific industrial equipment being controlled.
As industrial processes became more complex, a more elaborate sensing and control system was required. In many instances, the simple voltage or current input to and output from the programmable controller was insufficient to communicate the data to and from the processing equipment. In many instances, more sophisticated sensors were devised to provide detailed data about the individual workpieces being handled by the controlled equipment. For example, automobile assembly lines use radio frequency transponder tags mounted either on the automobile or the assembly line carriage for the automobile. As the automobile passes a work station, a transceiver interrogates the RF transponder tag to acquire data about the specific automobile being processed. Such data, for example, identifies the model of car as well as the options and features to be incorporated. Once the transceiver has interrogated the transponder tag, the data regarding the automobile is provided to a programmable controller at the work station over a serial communication link.
A large assembly line is typically controlled by a number of programmable controllers. In which case, serial communication networks also interconnect the programmable controllers allowing them to exchange data regarding the operation of the assembly line. In addition, a central host computer often is coupled to the network to receive assembly line status information from the programmable controllers and issue commands to them. In such installations, sensor data received by one programmable controller may have to be passed on to other programmable controllers or the host computer. For example, rather than locating an RF tag transceiver at every work station, the transceiver can be coupled to the programmable controller for one work station and that controller sends the data read from the tags onto other programmable controllers at adjacent downstream work stations.
Heretofore, serial communication modules for a programmable controller received data from a external device and supplied the data to a central processor module for the programmable controller. If that data was to be forwarded to another programmable controller or a host computer via another communication link, the central processor module had to devote some of its processing time to transferring the data back to the same or a different serial communication module which interfaced to that link. Since the primary role of the central processor module is executing a control program to operate the industrial equipment, handling communication tasks took time away from this role.
Furthermore, a sensing device may provide a long stream of data to the programmable controller, while only a small portion of the stream was needed to carry out the control function. For example, an RF tag system provides detailed information regarding the options to be incorporated in an automobile being assembled, whereas a given work station might only need the model designation. In previous systems, the processor module had to devote part of its processing time to parsing the RF tag data to obtain the model number. This parsing function also took processing time away from the primary machine control function.